An update on this for posterity. The cat pipe and the header pipe are either welded together or crimped (can't tell) and the cat pipe fits into a rather trumpet shaped flange in the header pipe. They completely and cleanly separated, I think just fatigue from 33 years. It's not rusted. I used a cutoff wheel to slice off that flange. The header pipe is a weird OD of course, like 2.7". Too big for a clean slip fit with a 2.5" ID and too small for a 3" ID. O'Reilly's actually has stainless exhaust parts, a shocker. I bought a 3" ID to 2.5" ID reducer, the cat side pipe is 2.5" OD and the reducer slid on perfectly and snugly. I had to also buy a piece that fit snugly inside the 3" side of the new exhaust reducer and cut a 1" wide portion to slip in there for a gasket/shim. 1.5" wide would have been better I think. This was enough to get a fairly tight, but not perfect, fit. The clamp for the header side needed a few washers to keep the threads from bottoming out with the size of the pipe, and this was a 3" clamp. They should have made the threads a little longer. Used the floor jack to gently push on the cat to bring the assembly up to a nice straight level. Electric impact to tighten the clamps really good. A touch of exhaust sealer and it's perfect now. Not too hard at all and maybe $50 max in parts.
